A recent development in third generation (3G) wireless communications is the long term evolution (LTE) cellular communication standard, sometimes referred to as 4th generation (4G) systems. Both of these technologies are compliant with third generation partnership project (3GPP™) standards. Irrespective of whether LTE spectral allocations use existing second generation (2G) or 3G allocations being re-farmed for fourth generation (4G) systems, or new spectral allocations for existing mobile communications, they will generally use paired spectrum for frequency division duplex (FDD) operation.
Referring to FIG. 1, an example of a simplified evolved packet system (EPS) 100 is illustrated comprising, part of an evolved packet core (EPC) network 101, access network 103 and user equipment (UE) domain 105. In this case, EPC network 101 comprises the Internet 128, a packet data network gateway (P-GW) 107, a serving gateway (S-GW) 111 and a mobility management entity (MME) 113. The P-GW 107 is situated between the Internet 128 and the S-GW 111, and may include responsibility for IP address allocation for UEs 115, as well as Quality of Service (QoS) enforcement and flow based charging according to rules of a policy control and charging rules function (not shown). The P-GW 107 is responsible for filtering of downlink user Internet Protocol (IP) packets into different QoS-based bearers. The P-GW 107 also serves as a mobility anchor for inter-working with non-3GPP technologies such as CDMA2000 and WiMAX™ networks. All IP packets are transferred from the P-GW 107 to the serving gateway (S-GW) 111 via an S5/S8 interface, which serves as a local mobility anchor for data bearers when user equipment (UEs) 115 move between base stations/eNodeBs 117. The S-GW 111 also retains the information about the bearers when the UEs are in idle state (known as EPS Connection Management IDLE (ECM-IDLE)) and temporarily buffers downlink data while the MME 113 initiates paging of the UEs 115 to re-establish the bearers. In addition, the S-GW 111 performs some administrative functions in the access network 103, such as collecting information for charging and legal interception. It also serves as a mobility anchor for inter-working with other 3GPP technologies such as general packet radio service (GPRS) and universal mobile telecommunications service (UMTS). The S-GW 111 is coupled to the MME 113 via an S11 interface.
Access network 103, defined by a number of inter-connected eNodeBs 117, is generally utilised when UEs 115 are in a network's coverage area 119, thereby allowing UEs 115 to communicate with each other solely via the access network 103. Generally, the access network 103 communicates with the EPC network 101 via S1-U 121 and S1-MME 123 interfaces. eNodeBs 117 are operable to communicate with each other within the access network 103 via X2 interfaces 125. In this case, UEs 115 are operable to communicate with eNodeBs 117 via a Uu interface, otherwise known as radio interface 127. In this case, access network 103 is utilised when UEs 115 are within the access network's 103 network coverage 119, allowing them to communicate with one another. Generally, the access network 103 facilitates communication by receiving control plane data and user plane data from each eNodeB 117 and from UEs 115, and transmitting this control plane data and user plane data to the other eNodeBs 117 within the access network 103. Different eNodeBs 117 within the access network 103 may utilise different receiving and transmitting frequencies, for example if Frequency Division Duplexing (FDD) is utilised. Further, different eNodeBs 117 within the access network 103 may utilise different waveforms, signal modulation and coding schemes between the different eNodeBs 117. Specifically, in a generic LTE system, referred to as E-UTRAN, the Uu radio interface 127 generally utilises Orthogonal Frequency Division Multiple Access (OFDMA) in the Downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the Uplink. OFDMA distributes subcarriers to different users (UEs) at the same time, allowing multiple users to be scheduled to receive data simultaneously.
Generally, subcarriers are allocated in contiguous groups for simplicity and to reduce any overhead of indicating which subcarriers have been allocated to each user. SC-FDMA is generally utilised in the Uplink case as it has a lower peak-to-average power ratio compared to OFDMA, which can benefit mobile terminal devices in terms of transmit power efficiency, for example. As discussed above, FDD may be utilised resulting in differing transmit and receive carrier frequencies. Further, Time Division Duplexing (TDD) may be utilised, resulting in separate outward and return signals.
A potential problem occurs when, for example, the P-GW 107 within the EPC network 101 fails. If this failure occurs, at least user plane data will be affected, and it may not be possible to access the packet data network 128 or route data to the UEs 115. Failure of the P-GW 107 may prevent public safety systems from offering services to local users. Therefore, in some cases, it may be desirable for public safety systems to be able to offer services to local users despite failures within the EPC network 101.